This invention relates to an injection molding system having at least one nozzle for feeding plasticized resin material into a mold cavity through an inlet runner, and a pressurized fluid conduit for distributing the resin on the walls of the mold cavity.
U.S. Pat. No. 4,101,617 and German published Patent Application No. 21 06 546 disclose methods for the injection molding of articles from thermoplastic resins. In the German application shoe heels are produced by first injecting the requisite quantity of plasticized resin from a nozzle into the mold cavity of a two-part mold, the nozzle being stationary relative to the mold. Compressed gas, such as compressed air, is then injected from a second nozzle into the center of the molten plastic of the mold cavity in order to distribute the plastic on the walls of the mold cavity. The second nozzle is separate and spaced from the first nozzle.
Upon resin solidification, the mold is opened whereby the second nozzle, which is arranged substantially perpendicular to the parting surface of the mold halves, and which is fixed to the mold and projects at a constant length into the closed mold, is removed from the shoe heel thus resulting in establishing a pressure equilibrium between the interior of the shoe heel and the surrounding atmosphere.
In the aforementioned U.S. patent, square hollow bodies such as glass bricks are produced by first injecting, by means of a concentric double nozzle, plasticized resin out of an outer annular nozzle into the mold cavity of a two-part mold, and thereafter injecting compressed gas out of the inner circular nozzle into the mold cavity. This approach can be applied both to a mold cavity with solid walls and to a mold cavity having a variable volume, and comprises a female and male mold. Finally, the single injection opening in the plastic mold can be plugged with additional plastic after solidification of the plastic in the mold cavity and prior to opening the mold, thereby requiring the plastic mold to be pierced or drilled for purposes of equalizing the pressure between the interior of the mold and the surrounding atmosphere. Otherwise, the pressure can be equalized by simply pulling back the concentric injection nozzle from the mold.
In both the above prior art systems it is possible to advance the nozzles directly as far as the mold cavity or to use at least only very short inlet runners in the mold body, and to provide compressed gas nozzles directly into the mold cavity. Thus, the plasticized resin flows from the plasticized unit into the mold cavity substantially without any cooling, whereafter the cooling and resulting solidification on the cavity walls commences.
With the mold cavity having a sufficiently large volume the injection pressure of the plasticized resin and thereafter the use of compressed gas to distribute the plastic uniformly over the walls of the mold cavity, prior to its solidification, is known. For large mold parts having a large area or only a relatively small cross-sectional area in some locations thereof, the flow resistance of the plasticized resin in the mold cavity is so great that at admissible injection pressures and temperatures the plastic cools and solidifies on the way through the mold cavity in its regions that are further away from the nozzle, before the plastic even reaches such regions. This condition cannot be corrected even with an increase of the injection pressure of the compressed gas, a feature that leads to gas breakthroughs in the walls of the incomplete plastic mold.
U.S. Pat. No. 4,942,006 discloses a system of inlet runners from a separate injection nozzle to a plurality of nozzle openings which open into individual mold cavities and which then stress each mold cavity at one or several locations simultaneously with plasticized resin. The mold having several mold cavities can be arranged side-by-side, and molds having a single mold cavity can be utilized as having a large cavity or large volume which must be supplied with plasticized resin at several points simultaneously. In order that the plasticized resin does not cool and solidify during its path of travel, which is significantly longer, from the injection nozzle to each mold cavity so that at least the production of the next plastic mold is not adversely affected, each runner is normally provided with its own heater in the mold such that all the runners form a hot runner system which maintains the requisite ease of flow of the plasticized resin as it enters the mold cavity.
A mold cavity is pressurized with pressurized fluid, for example, a compressed gas, either contemporaneously with or following the injection of the plasticized material, via a concentric needle valve, as disclosed in the U.S. Pat. No. 4,942,006. And a plurality of inlet valves can be provided which are shifted toward and away from or are fixed and totally separated from the injection nozzle openings, as disclosed in European Patent No. 127 961 and German published Patent Application No. 38 34 917.
The drawback with each of the aforementioned arrangements is that they require extensive equipment and/or control engineering measures to effect a balance between the inlet of the plasticized resin and the pressurized fluid since both components do not join until they reach the mold cavity or until shortly before reaching the mold cavity, whereby one component penetrates rearwardly into the feed region of the other component thereby possibly producing a foaming of the resin. Otherwise, such could have a negative impact on quality of the molded part, particularly on its surface finish, or even cause breakdowns in the valve arrangements at the junction of the feed systems of both components with subsequent time consuming expenses for cleaning and maintenance work.